Sensors in the field of process measurement technology are subject to a strong application-related aging over time. Especially sensors used in liquid media, for instance for monitoring chemical processes. They are subjected to special demands, so that the requirements for their chemical and thermal resistance are high. Likewise, fouling and accretion formation on the sensor in the course of media contacts can interfere with its effectiveness and decrease its service life. The ability of a sensor to function and its service life are impaired, respectively influenced, by external factors or internal factors present in the sensor. It has, consequently, not been possible, to state, or predict, the service life of a sensor, neither in general, nor even for sensors in special cases of application. This situation is clearly disadvantageous.
In the case of electrochemical, electrophysical or optical sensors in the field of process measurement technology, calibrations are usually performed from time to time, wherein a two, or more, point calibration is established by means of appropriate standards. The calibration data obtained in this way are stored in a memory of a measured value transmitter or in the sensor itself (see the as yet unpublished DE 102 18 606.5) to be available for the measured value evaluation. During the registering and evaluation of a measured value, these calibration data, and perhaps other measured data, such as e.g. the temperature of the medium being measured, are then referenced.
For example, a pH-measurement chain, formed by a pH-glass-electrode and a reference-electrode, is characterized by the parameters zero-point and slope of the measurement chain voltage curve. Known glass electrode errors are slope error, alkali error and zero-point error. The service life of a pH-electrode is, consequently, dependent both on the conditions of use and on the durability of the electrode glass being used. As a rule, influences on the so-called membrane surface, thus accretions of lime, gypsum, fats, proteins and the like, can be removed by chemical cleaning. Not removable, however, are aging processes within the sensor, which occur even when the sensors are merely being stored. Above all, however, use at extreme pH-values or high temperatures decrease the service life of a glass electrode drastically.
Reference electrodes, in contrast, have a diaphragm as the site for the electrolytic contact between a reference electrolyte and the solution being measured. Errors can be caused, consequently, for example by fouling and blocking of the diaphragm or by so-called electrode poisoning due to intruding foreign ions in the reference electrolytes. The result is a changing of the cell voltage of the reference electrode (the half-cell voltage), of the so-called reference potential, which, in turn, shows up as a change in the zero point of the total measurement chain voltage.
There are, therefore, methods for monitoring the functioning of electrochemical sensors, wherein, for example, the internal resistance of a measuring electrode and a reference electrode is measured at timed intervals and, upon the exceeding or subceeding (falling beneath) of values determined in practice, an alarm is issued. According to this method, one waits until the sensor has reached a critical stage and must then be replaced.
There is a multitude of methods for monitoring the functioning of sensors in the field of process measurement technology, wherein the instantaneous successful functioning of sensors is checked, always on the basis of test parameters. DE 42 12 792 C2 and DE 34 19 034 are named by way of example.